The present invention relates to pitch having excellent properties for the production of carbon fibers.
There is known a method of producing carbon fibers of high strength and high modulus of elasticity in which pitch is subjected to heat treatment to obtain meso-phase pitch containing from 40 to 100% of optically anisotropic liquid crystals and the meso-phase pitch is then subjected to melt spinning, infusiblization and carbonization (for example, see Japanese Patent Laid Open No. 19127/1971).
It is also known that if pitch with a meso-phase content of 40% or lower heretofore known is employed, there will takes place separation of the optically anisotropic and isotropic regions, and the melt spinning would result in frequent end breakages and, in extreme cases, production of fibers in which undissolved particles are connected. Treatment of such fibers by a conventional method is known not to produce carbon fibers having high strength and high modulus of elasticity.
Therefore, most of the prior-art processes are directed to a method of producing carbon fibers having high strength and high modulus of elasticity by the use of pitch containing from 40 to 100%, particularly from 70 to 100% of meso-phase pitch. Almost no attempts have been made to produce carbon fibers having high strength and high modulus of elasticity by using pitch of a meso-phase content of 40% or lower.
Pitch having a higher meso-phase content, however, is very high in softening point as well as in viscosity and usually requires melt spinning carried out at a temperature as high as 350.degree. C. or higher. Consequently, pyrolysis and heat polymerization of the pitch are apt to occur in the course of melt spinning with such problems as evolution of light gases and formation of infusible components associated so that homogeneous spinning is difficult to accomplish.
Moreover, carbon fibers obtained from pitch of a high meso-phase content have a tendency to form so-called radial structure which is composed of crystals radiately arranged on the cross ection. Such structure causes a problem of longitudinally generating cracks to reduce the strength.